Semi-permanent unnatural hair colors?

And by semi-permanent I mean really, like lasts for a while but won't stain your hair and it goes back to the colour underneath. I have light blond hair, but I like it that way but I also want to have fun with colours as I've never been one to stick to one colour for too long. I just want a colour that I can have fun with, but change if I want too and not have to make a huge commitment. Also if I decide to change my mind and just fancy being blond, that it won't leave a funny coloured stain. Any recommendations? thanks :)